Variable resistance materials are promising active materials for next-generation electronic storage and computing devices. A variable resistance material possesses two or more states that differ in electrical resistance and can be programmed back and forth between the states by providing energy to induce an internal chemical, electronic, or physical transformation of the material that manifests itself as a change in resistance of the material. The different resistance states can be associated with different data values and used as memory states to store or process data.
Phase change materials are a promising class of variable resistance materials. A phase change material is a material that is capable of undergoing a transformation, preferably reversible, between two or more distinct structural states. The distinct structural states may be distinguished on the basis of, for example, crystal structure, atomic arrangement, order or disorder, fractional crystallinity, relative proportions of two or more different structural states, or a physical (e.g. electrical, optical, magnetic, mechanical) or chemical property. In a common embodiment, the two or more distinct structural states include differing proportions of crystalline phase regions and amorphous phase regions of the phase change material, where the phase-change material is reversibly transformable between the different states. In the crystalline state, the phase change material has lower resistivity; while in the amorphous state, it has higher resistivity. Continuous variations in resistivity over a wide range can be achieved through control of the relative proportions of crystalline phase regions and amorphous phase regions in a volume of phase-change material. Reversibility of the transformations between structural states permits reuse of the material over multiple cycles of operation.
Typically, a variable resistance device is fabricated by placing the active variable resistance material, such as a phase change material, between two electrodes. Operation of the device is effected by providing an electrical signal between the two electrodes and across the active material. In a common application, phase-change materials may be used as the active material of a memory device, where distinct data values are associated with the different structural states and each data value corresponds to a distinct resistance of the phase-change material. The different structural states employed in memory operation may also be referred to herein as memory states or resistance states of the phase-change material. Write operations in a phase-change memory device, which may also be referred to herein as programming operations, apply electric pulses to the phase-change material to alter its structural state to a state having the resistance associated with the intended data value. Read operations are performed by providing current or voltage signals across the two electrodes to measure the resistance. The energy of the read signal is sufficiently low to prevent disturbance of the structural state of the phase-change material.
In order to expand the commercial opportunities for phase-change memory, it is desirable to identify new phase-change compositions, device structures, and methods of programming that lead to improved performance. A key performance metric for phase-change memory is cycle life, which is a measure of the number of times the device can be reversibly programmed between memory states. Over the course of operation, a memory device undergoes multiple read and write cycles. In a binary memory device, the write cycles entail repeated transformations between two memory states. In a multilevel memory device, the write cycles entail repeated transformations between three or more memory states. In practice, the number of cycles of operation of a phase-change memory device is limited and after a certain number of cycles, the device fails. Various failure modes have been identified for phase-change memory devices including open circuit failure, short circuit failure, stuck set failure, and stuck reset failure.
Factors that may contribute to device failure include the phase-change alloy composition, electrode materials, interface contamination, and volatilization. The phase-change alloy composition is relevant because the constituent elements of many phase-change materials are capable of forming multiple stoichiometric compositions or multiple crystallographic structures. This tendency promotes the creation or stabilization of multiple stoichiometric or crystallographic phases during cycling. As a result, the repeated heating and cooling steps required for programming a phase-change material may induce phase segregation and/or a progressive evolution of the phase-change material to a less effective or non-functional structural or compositional state.
The choice of electrode material influences cycle life through the quality of the contact formed with the phase-change material. Consistent performance of the device over many cycles requires good adhesion of the electrodes to the phase-change material. The electrode material must be matched to the phase-change composition to achieve good adhesion. Poor adhesion may lead to delamination of the electrode from the phase-change material, which may lead to an open circuit condition.
Interface contamination can contribute to device failure by promoting phase segregation or the formation of non-functional phases. Oxygen, for example, is a common contaminant in device fabrication and is known to form stable insulator phases with elements, such as Ge, that are commonly present in phase-change compositions. The formation of oxides from the constituent elements of a phase-change composition may be promoted by the high temperature conditions associated with device programming. As a result, the composition of the phase-change alloy may evolve over time and transform to a non-functional state. The creation of a thick oxide layer at the electrode interface, for example, may prevent programming.
The high temperature conditions associated with programming may also promote volatilization of the phase-change material. Programming of a phase-change material to an amorphous state entails providing sufficient current to heat the phase-change material to its melting temperature. While in a molten phase, the phase-change material may experience evaporation. Similarly, sublimation may occur in a solid phase-change material upon heating. Since different elements of a phase-change material tend to evaporate or sublime at different rates, escape of elements due to volatilization alters the composition of the phase-change material. This effect is cumulative upon cycling. In addition to altering phase-change composition, the loss of material that accompanies volatilization creates a void in the working region of the device that impair electrical contact with the electrodes. As a result, delamination or open circuit failure become more likely.
There is a need in the art to develop phase-change materials that exhibit long cycle life. To be commercially successful, phase-change devices need to exhibit stable performance over prolonged time periods and multiple operating cycles. For many commercial applications, a phase-change memory device needs to perform for at least 106 cycles without failing. Ideally, it would be desirable to develop phase-change memory devices that are consistently stable over at least 109 operating cycles.